This invention relates to electrode materials, electrodes formed from such materials and electrochemical cells. It has particular reference to electrodes forming anodes and/or bipolar electrodes.
There are numerous requirements in electro-chemical reactions for electrodes capable of passing an electrical current when connected anodically without dissolving or passivating. Examples of electro-chemical reactions requiring the use of anodes include cathodic protection, the formation of chlorates and hypochlorites by electrochemical oxidation of a chloride solution, the formation of persulphates and perborates, metal winning, organic oxidations and the evolution of chlorine from a chlorine cell.
Initially anodes were made from graphite or platinum or materials such as silicon-iron for use in cathodic protection systems. Improved anode materials were discovered, which basically comprised a substrate of titanium having a coating of a platinum group metal. Later discoveries, including the use of platinum group metal oxides, were more durable and economic in certain instances. By the term "platinum group metal" as is used herein is metal chosen from the group ruthenium, rhodium, palladium, iridium and platinum. Platinum group metals are electrocatalytically active, by which is meant that the material, when connected as an anode and immersed in an electrolyte, will continue to pass current into the electrolyte. There may be the evolution of oxygen, as in the case of an aqueous sulphuric acid electrolyte, or chlorine, as in the case of aqueous chloride containing electrolytes, or an electrochemical reaction at the anode which does not involve the evolution of gas. By comparison titanium uncoated with an electrocatalytic material passivates when connected as an anode in aqueous solutions by the rapid formation of an oxide layer by anodic oxidation. This oxide layer acts as a barrier to further conduction of electricity between the electrolyte and the titanium substrate. The third type of anodic reaction which can occur is for dissolution of the anode material, such as happens, for example, when copper is used as an anode in an aqueous copper sulphate solution.
Although anodes comprised of a sheet of titanium, in solid or foraminate form, having an electrocatalytic coating thereon have proved commercially feasible and are in widespread use, the cost of titanium does adversely affect the economic viability of the anodes. Also, titanium does suffer from a number of problems when used as a substrate for an electrocatalytic coating. Titanium can be subject to acid attack leading to acid undermining of the anodically active coating and the coating thereby falling off. Also titanium is rapidly hydrided when it is in the presence of hydrogen. Titanium hydride is brittle and falls into a powder.
The present invention is concerned with an electrode material capable, in the right circumstances, of use in electrochemical reactions in the uncoated state or, in alternative arrangements, for use in electrochemical reactions in the coated state. The present invention also provides an electrode formed of such an electrode material and an electrochemical cell incorporating electrodes formed from such electrode materials.
British Patent Specification No. 232 680 describes the manufacture of partially reduced titanium oxide. However, the techniques described, namely heating in air at temperatures between 800.degree. and 1000.degree. C., will not permit the reduction of the titanium oxide to any significant extent. Even reducing in hydrogen at these temperatures would only permit an x value in TiO.sub.x of approximately 1.93 to 1.94.
British Patent Specification No. 1 231 280 proposes the use of substoichiometric titanium dioxide as coatings for titanium anodes. These substoichiometric oxides are regarded as the anodic coatings in their own right. British Patent Specification No. 1 443 502 describes the use of a three-layer electrode of titanium in which there is an intermediate layer of TiO.sub.x wherein x is between 0.25 and 1.50 with an outer anodically active layer. British Patent Specification No. 1 438 462 describes an electrode comprising a flame sprayed or plasma sprayed layer of titanium suboxide on a substrate such as titanium wherein the titanium suboxide has the formula TiO.sub.y where y is between 0.1 and 1.999.